A process for making polycrystalline silicon may include feeding gases comprising hydrogen and a silicon monomer (such as HSiCl3 or SiH4) to a fluidized bed containing silicon particles that are maintained at high temperature. The particles grow in size, and when large enough, are passed out the bottom of the fluidized bed reactor (FBR) as product. The vent gases exit the top of the FBR. The vent gas stream can be passed through a recovery process where condensations, scrubbing, absorption and adsorption are unit operations often used to facilitate the capture of silicon monomer and hydrogen for recycle.
One problem with the FBR process is that the wall surrounding the bed of silicon particles must be heated to a temperature higher than the average bed temperature to facilitate heat transfer. That can be done, for example, by use of resistance heating, microwave energy, radio frequency inductive heating, or infrared radiation. All heating methods have unique operating problems. One problem, however, is that the bottom of the FBR may be hot, and the feed gas is reactive when it contains HSiCl3 and hydrogen. As a result, the feed gas distributor, clusters of large particles, especially those in regions where agitation is less active and particles are in contact for prolonged periods of time, and reactor side walls are prone to rapid deposition of silicon. Those deposits subsequently disrupt the proper feed distribution, product separation, and heat transfer of the system.
Another problem with the FBR process is that slugging can occur in a cylindrical FBR because bubbles grow quickly in fluidized beds containing relatively large particles. “Slugging” refers to formation of gas bubbles that are so large that they disrupt fluidization and reduce yield. Typically, these bubbles can reach a diameter close to a diameter of the vessel. The act of slugging leads to significant pressure fluctuations in the system and uneven forces on the vessel wall. Since the fluctuation behavior is dynamic with bubbles growing in size and breaking apart at the freeboard, the preferred approach for characterizing its magnitude is the time series-average of the standard deviation of pressure fluctuations. The standard deviation of pressure fluctuations is proportional to the excess gas velocity and hence a measure of aggregate bubble size for the largest bubbles formed in the system. These forces manifest themselves with measurable vibrations, and tools exist in the art to measure such vibrations in the field. For instance, the pressure drop across the entirety or a section of the fluidized bed can be measured using differential pressure transducers.
A third challenge with silicon deposition fluidized bed reactors is the delivery of energy into the process to support the deposition chemistry if the wall is the primary mode of transport. As diameter of a cylindrical FBR increases, the wall perimeter of the FBR increases linearly and hence the wall surface area per unit length increases linearly, but the heat requirement for operation increases with the square of diameter of the FBR at a given average superficial velocity. To accommodate for this change, either the heat flux needs to increase or the bed height must increase to achieve the necessary delivery of energy. The maximum heat flux from the wall to the bed can be limited by the allowable thermal stress associated with the wall material of construction and heat delivery method. Internal heaters and heat exchangers have been proposed in the art to supplement energy input, but these add complexity not to mention challenges with maintaining product purity. Height is also limited by growth of bubbles for some fluidization processes. With limitations on heat flux, increases to the FBR diameter result in larger height, but bubble growth rate increases resulting from more bed level can lead to excessive slugging in the FBR. This makes it difficult to scale up an FBR process into a commercially viable producer of polycrystalline silicon.
Polycrystalline silicon particles made in fluidized bed reactors generally fall into Geldart Group B and/or Geldart Group D classification. The Geldart Classification Scale refers to the collective behavior of particles in fluidized bed reactors, and distinct regimes of this solid behavior can be characterized from the average particle size and the relative densities of the solid and fluid phases. For instance, the appropriate averaging description for the particle size distribution that adequately quantifies the drag behavior of a gas-particle system is the surface-to-volume mean or Sauter mean particle diameter. This metric shows the influence of gas-solid drag from the smaller particles on the overall population which helps move larger beads. For silicon particles, Geldart Group B particles may have particle diameters ranging from 200 to 800 micrometers. Geldart Group B particles exhibit formation of bubbles at the onset of fluidization, which continue to grow from the point of injection. The bubble size can be large, e.g., on the order of feet in some cases. For the silicon deposition FBR processes, samples of industrially available material show particle size distributions to fall within the Geldart B range, typically 700-800 micrometers.
Geldart Group D particles have the largest particle diameters of all Geldart groups. Gas requirements for fluidization of Geldart Group D particles are larger than Group B particles and bubble growth is also faster. During fluidization, Geldart Group B and Geldart Group D particles have enormous bubble diameters, and spouting and/or slugging are commonly observed in large, cylindrical fluidized beds. Due to the problems of slugging, Geldart Group D particles are typically processed in spouting beds where gas requirements are less than needed in bubbling fluidized beds, but use of a central feed jet limits the efficiency of gas-solids contact. Alternatively, a method to reduce bubble growth within bubbling fluidized beds is to induce breakage of bubbles within the bed. While mechanical solutions such as baffles can be used to break bubbles, in the polysilicon deposition application, implementation of a robust, erosion-resistant baffle design that likewise doesn't induce contamination of product is difficult.
A fourth complexity of silicon deposition fluidized bed reactors is the tendency to produce fines (on the order of 1 micrometer particle size) within the reaction system that entrain out of the reactor. Chemistry within the bubble phase is also known to facilitate nucleation reactions that create the fine silicon powders. A common method to limit this problem is to shrink the average particle size within the bed to limit growth of bubbles. Smaller particles however have a higher likelihood of contamination risk. Alternatively, fluidized bed deposition reactors can be operated in series but this requires more capital investment.
There is a need in the polycrystalline silicon industry to improve FBR technology to address these problems.